Th. 3e cycle--Electronique, électrotech. autom.--Toulouse 3, 1981. N°: 2479.

No description available.

A collaborative research project was set up to study peripheral diamond wheel machining damage in silicon nitride ceramics. The objective of the work to be carried out at the University of Surrey was to study the nature and depth of machining damage in 3 point flexural rupture test bars made from 5 wt% Y2O3 sintered hot isostatically pressed silicon nitride. The bars were machined to three surface finishes. The work carried out at Rolls-Royce by Mr R Quinn concentrated on the effects the machining damage had on the fracture strengths of the test bars. Work at the University of Surrey has identified in detail the nature and depth of machining damage in "coarse" 0. 4mum centre line average roughness surfaces. Deep grooves up to 2mum depth, 18mum width are found to have been superimposed on the general surface roughness by singularly large diamonds in the 350 grit diamond wheel. Sub-surface median cracks normal to the machining direction were clearly identified in bar cross-sections using oblique, diffuse "penumbra" illumination in an optical microscope, an as yet undocumented technique. Cross-section views of the machined surface and sub-surface were made possible by the very difficult and delicate technique of producing sandwich cross-sections of the machined bars. An analysis of machining-induced median cracks has not been carried out in such detail before. Semi-elliptical in shape the median cracks extend from 6mum - 45mum below the machined surface, and range from 19mum to 101mum in length parallel to the machining direction. They initiate at the focal point of a tributary system of microcracks at an average depth of 4mum - 5mum below the machined surface. It is believed that the median cracks initiate at the plastic/elastic boundary of a plastically deformed surface layer. Therefore a residual compressive layer, formed by the overlap of localised residual stresses from multi-particle contact events. and bound by an underlying tensile field, is thought to have an average depth of 4mum - 5mum. A very innovative technique was used to reveal sub-surface deformation, where TEM X-ray microdiffraction spots were distorted by mechanical damage in the ceramic grain structure. The "arcing" or "streaking" of the diffraction spots tended to disappear at a depth of 4mum - 5mum below the machined surface. This is further evidence of the existence of a thin layer in residual compression, which has an average depth of 4mum - 5mum. This technique is not known to have been used before. Fine diamond machining with a 600 grit wheel produced a centre line average roughness of 0.01/0.02 mum. However, evidence of machining damage is still present in the form of "remnant tracks" which lie parallel to the machining direction and consist of material pull-out. They are remnants of machining damage under grinding grooves introduced in previous machining stages. Single point Vickers pyramid diamond scratches were implemented at different loads on a polished surface. The morphology of the grooves and material fragmentation and the sub-surface median cracks were examined. Many features were found to resemble the deformation/fracture formed under a deep grinding groove in the coarse machined surface. Work carried out at Rolls-Royce by R Quinn showed that an increase in the quality of surface finish is accompanied by an increase in the mean strength and Weibull modulus of the machined bars. Furthermore a distinct anisotropy in the fracture strengths parallel and normal to the "coarse" machining direction is evidence of anisotropy in machining damage formed by a peripheral diamond grinding wheel. X-ray diffraction tests carried out at the CEGB by P E J Flewitt showed that machining damage produces a long range biaxial residual compressive field with the highest component acting normal to the machining direction. These results are consistent with the nature of machining damage identified at the University of Surrey, namely the strength-controlling median cracks which lie parallel to the machining direction and the residual compressive stress which exists as a thin 4mum - 5mum layer below the machined surface. Processing flaws were discovered in the as-hipped billets received for the project. Their elemental composition and likely origin were examined. A three dimensional "cellular network" flaw ranging from 400mum to 2.1mm in size (in different production batches) is believed to have been formed as a result of flocculation clustering during processing. Clusters of 1mum - 3mum metallic particles were also identified. They range from 5mum - 45mum in size. The contaminant particles are steel and were introduced as a result of the original ceramic powder ball milling process which employed a steel ball mill.

No description available.

Desalination plants are becoming essential due to the limited water resources in order to reduce the pressure of high demand of freshwater in many countries in recent decades. A concerning problem associated with desalination plants is the high concentration brine which has high risk to marine environments. Inclined dense jets are commonly used to treat brine produced by desalination plants or in industrial outfall discharges. They are produced when the brine is discharged at an upwardly inclined angle through a pipe or a diffuser system. Previous studies have mainly focused on jets on a horizontal bottom. In the present study, the influence of sloped bottom is investigated by numerical simulations using a modified solver in OpenFOAM (pisoFoam). Four different Reynolds Averaged Navier Stokes (RANS) turbulence models (Realizable k-ε, Standard k-ε, RNG k-ε and Nonlinear k-ε were employed to assess the accuracy of the selected turbulence models in predicting the jet behavior. Jets of inclination angle of 30° with four different initial conditions (Froude number=15, 20, 25, 30) on three different bed slope angles (2°, 5°, 10°) in stagnant water were conducted. Although inclined dense jets of the discharge angel of 60° are more common in discharge systems, sometimes they cannot be used in shallow waters in order to prevent surface pollution. In such cases, a relatively small jet inclining angle can be used to prevent the surface pollution and as shown in this thesis, bed slope can enhance the brine mixing and dilution. The results showed that Realizable k-ε model is more accurate among the turbulence models studied herein. The dilution at the impact point can be estimated based on the Froude number and initial conditions. After the impact point, the slope did enhance the dilution of the plume compared to the horizontal bed. The dilution was thus affected by the slope and the dilution after the impact point on the slope appeared to be linearly related to the distance to the source. Besides, the slope could enhance the jet dilution up to 20% compared with the horizontal bed after the impact point.

Cette thèse étudie la stabilité linéaire et le contrôle linéaire optimal d’écoulements périodiques en temps. Le cadre d'étude développé a été appliqué au jet rond pulsé.Lorsqu'un jet rond laminaire est forcé au niveau de sa buse périodiquement et de façon axisymétrique, une allée régulière d'anneaux de vorticité se forment. Dans cette configuration, nous étudions deux types d’instabilités. D'une part, de façon intrinsèque, un appariement tourbillonnaire peut parfois survenir. D'autre part, l'ajout d'un terme hélicoïdal sous-harmonique au forçage axisymétrique peut générer un jet bifurqué. Ces deux phénomènes conduisent à une importante augmentation de l'évasement du jet et à une amélioration de ses propriétés de mélange.Tout d'abord, nous présentons une méthode de stabilisation d’orbites périodiques instables, basée sur un contrôle avec un retard temporel. Cette technique, appliquée au cas du jet pulsé, fournit un écoulement périodique non-apparié, dans des gammes de paramètres où l'appariement surgit naturellement. Cet écoulement non-apparié forme la base des études de stabilité et d’optimisation suivantes.Ensuite, nous étudions la dynamique intrinsèque des perturbations de cet état. D'une part, grâce à la théorie de Floquet, nous calculons sa stabilité modale, ce qui prédit le comportement à long-terme de ces perturbations. Pour ce faire, une base de Krylov est construite à l'aide d'une méthode d'Arnoldi par blocs à partir de simulations temporelles. D'autre part, nous caractérisons sa croissance transitoire, qui contrôle le comportement à court-terme des perturbations. Tandis que l'analyse de Floquet prédit avec précision les nombres de Reynolds et de Strouhal critiques pour observer une croissance modale sur le long terme des perturbations puis un appariement, la croissance non-modale contrôle entièrement la bifurcation entre l’état non-apparié et l’état apparié.Enfin, nous optimisons le déclenchement de la bifurcation du jet. Ainsi, le forçage hélicoïdal maximisant l'évasement et le mélange du jet dans un plan préférentiel est calculé. Nous comparons ensuite par simulation numérique directe l'efficacité de ce forçage avec des forçages ad hoc utilisés dans des études précédentes. Le forçage optimal déclenche la bifurcation beaucoup plus tôt, avec un évasement bien plus marqué, et pour une gamme de nombres de Strouhal bien plus large que les forçages précédents. / This thesis describes the linear instability analysis and the design of linear optimal control of time-periodic flows. The numerical framework developed is applied to the study of pulsed jets.When a laminar round jet is forced axisymmetrically and time-periodically at the inlet, a regular street of vortex rings is formed. Two instability phenomena of such arrays are investigated. Firstly, intrinsic mechanisms may trigger vortex pairing. Secondly, if an additional subharmonic helical component is superposed onto the fundamental axisymmetric forcing, jet bifurcation is induced. Both phenomena result in strongly increased spreading and mixing in the mean flow.In a first step, a numerical stabilisation technique is devised, allowing the computation of exact periodic flow solutions, even when they are subject to instrinsic instabilities. This method, based on a time-delayed feedback, is then applied in order to recover unpaired periodic flow states of pulsed jets, in parameter regimes where vortex pairing naturally occurs. These unpaired flow states form the basis for the following instability and optimal control calculations.In a second step, the instrinsic perturbation dynamics in pulsed jets is investigated. Modal instability properties, governing the long-time flow behaviour, are examined in the framework of Floquet theory. Numerically, a Krylov basis is constructed from linear time-stepping using a block-Arnoldi algorithm to maximise efficiency. Transient dynamics, governing the short-time growth of initial perturbations, are characterised by an optimal perturbation analysis. While the modal Floquet analysis accurately predicts the critical Reynolds and Strouhal numbers of the long-time occurrence of vortex pairing, transient growth dynamics dominates the bifurcation.Finally, the optimal way to trigger jet bifurcation through subharmonic inlet forcing is computed. Inlet helical forcing is identified that maximises the jet spreading and mixing in one privileged meridional plane. This optimal forcing is implemented in direct numerical simulations, and its efficiency in the nonlinear regime is compared to that of ad hoc forcing used in previous studies. The optimal forcing results in bifurcation further upstream, at higher spreading angles, and over a much wider range of Strouhal numbers than found previously.

No description available.

No description available.

This Research explores hydrogen combustion in a Jet-in-Crossflow configuration through computational fluid dynamics using ANSYS Fluent commercial CFD software. Three fuel-only hydrogen jets with a momentum flux ratio J of 10, 50, and 115 were introduced axially, using a large eddy simulation with a WALE sub grid model. Detailed chemistry was computed directly with a 9 species hydrogen/air kinetic mechanism. The 4mm jet and crossflow domain utilized an automatic mesh adaptation method centered around the flame shear layer. The study models the second stage of a lab-scale gas turbine test facility at a pressure level of 5atm,a crossflow temperature of 1620K, and crossflow velocity of 75m/s. The models were compared to physical experiments conducted and analyzed with line-of-sight CH* chemiluminescence to create more insight into the phenomena of the combustion process. Flame position along the windward and leeside stabilization points were overlaid, and the validated CFD model utilized to characterize reaction progress as a function of jet entrainment with hot oxidizer. At elevated momentum flux ratio, increased reaction rates along the shear layer of the diffusion flame were attributed to the enhanced contact area between the fuel jet and crossflow oxidizer. The results outline the potential of carbon-free combustion technology and highlight the importance of tuning the operating condition for application in gas turbines.

The prediction of the flows and temperatures in the moderator system of CANDU reactors is important in the safety evaluation during some potential transients. An experimental program to collect data for CFD validation, including integrated moderator circulation tests has been funded by the CANDU Owners Group. This thesis constitutes a separate effect test within this larger moderator flow study, investigating the behaviour of a jet impinging onto a single heated cylinder. A number of experiments were conducted to investigate the behaviour of the jet flow under a variety of scenarios. The inlet Reynolds number and the heater power level were the primary variables considered to assess the impact of the buoyant forces generated by the heated cylinder on the flow. Alongside the experiments, simulations were performed using the experimental geometry to evaluate the performance of some of the most commonly-used turbulence modelling approaches – namely the standard k-ε, realizable k-ε, and standard k-ω RANS models. The agreement between the turbulence models and experimental results was determined to be reasonable in the free jet regions, however nearer the cylinder, the simulated results exhibited a wider core region and steeper gradient in the shear layer than the experimental data. At lower Reynolds numbers, over-prediction of velocities both in the axial and lateral direction was also seen. The impact of heating proved minimal in the jet core, however differences were observed in the shear layer at lower Reynolds numbers, and the heated case exhibiting decreased lateral velocities as compared to the isothermal case. / Thesis / Master of Applied Science (MASc)